1. Field of the Invention
The present invention relates to a method and apparatus for ultrasonic measuring. More particularly, the present invention relates to a measuring method and apparatus that can accurately estimate the propagation distance or propagation direction of an ultrasonic wave by using a spread spectrum coded ultrasonic wave even when there is a relative velocity between the object of measurement and the measuring apparatus.
2. Description of the Related Art
An ultrasonic measuring apparatus includes a transmitter to send out an ultrasonic wave and a receiver to receive the ultrasonic wave and estimates the distance between the transmitter and the receiver by the amount of time it has passed since the transmitter sent out the ultrasonic wave and until the receiver receives the ultrasonic wave. Alternatively, another ultrasonic measuring apparatus may estimate the distance between an object and the ultrasonic measuring apparatus itself by the amount of time it has taken for an ultrasonic wave, sent out from the transmitter, to reach the object, get reflected by the object and then get received at the receiver.
In an environment where there are a number of such ultrasonic measuring apparatuses, the ultrasonic waves transmitted simultaneously by the respective ultrasonic measuring apparatuses would interfere with each other, thus possibly causing measurement errors. To avoid such a situation, somebody proposed a method for distinguishing the ultrasonic waves from each other by coding the ultrasonic waves, generated by the respective ultrasonic measuring apparatuses, with mutually different codes.
A conventional ultrasonic measuring apparatus that adopts that coding method is disclosed in Japanese Patent Application Laid-Open Publication No. 2004-108826, for example. FIG. 11 is a block diagram showing the conventional ultrasonic measuring apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 2004-108826. Hereinafter, the basic operation of this conventional ultrasonic measuring apparatus 101 will be described. The ultrasonic measuring apparatus 101 includes a transmitter 8, a receiver 9, a correlator 103, a peak detector 104 and a pulse generator 105.
The pulse generator 105 generates a drive signal for the transmitter 8, which sends out an ultrasonic signal into the space. The ultrasonic signal transmitted passes through an ultrasonic wave propagation path 7 to reach an object 3 and get reflected by the object 3. The ultrasonic signal reflected passes through the ultrasonic wave propagation path 7 again to reach the receiver 9. The drive signals are coded with mutually different codes by respective ultrasonic measuring apparatuses so as to be identified from each other even when the ultrasonic signals sent out by those apparatuses interfere with each other. Considering such a situation where a desired signal should be decrypted and extracted from a number of signals interfering with each other, the other signals are preferably quite dissimilar from the desired one. Random signals that are generated artificially under a predetermined rule so as to have such a characteristic are called “pseudo random signals”.
Digital signals represented as a combination of “1” and “0” or “1” and “−1” are often used as the pseudo random signals because digital signals are easy to process. Examples of known digital pseudo random signals include an M-sequence, a Barker sequence and a Golay sequence. Among other things, the M-sequence functions as a code for use in a telecommunications system that adopts the spread spectrum technology. That is to say, the M-sequence is no different from noise with respect to the information to be transmitted but can function as an identifiable carrier when subjected to correlation processing using pulse compression. One of two different M-sequences looks nothing but noise for the other. That is why it is very effective to extract its own signal. Also, even if there are two identical M-sequences, one of the two also looks nothing but noise for the other when there is even a slight time lag between them. As a result, by coding the transmitted signal with the M-sequence pseudo random signal, a desired received signal can be extracted from a time series of received signals that interfere with each other and the time of its reception can be determined.
The drive signal generated by the pulse generator 105 is a spread spectrum (M-sequence discrete) random wave. Japanese Patent Application Laid-Open Publication No. 2004-108826 realizes a pseudo random signal with such a characteristic by binary frequency shift keying in which the frequency associated with bit one and the frequency associated with bit zero are different from each other.
The ultrasonic wave that has left the transmitter 8, passed through the ultrasonic wave propagation path 7 and then reached the receiver 9 has its correlation with the pseudo random signal, generated by the pulse generator 105, examined by the correlator 103. The peak detector 104 detects the peak of the correlation value. The time when the correlation value reaches its peak represents the time when the ultrasonic wave, sent out from the transmitter 8, reaches the receiver 9. And the interval between the time when the ultrasonic wave was transmitted and the time when the correlation value reaches its peak represents the propagation time of the ultrasonic wave to the object 3. Consequently, the distance from the ultrasonic measuring apparatus 101 to the object 3 can be measured by the propagation velocity of the ultrasonic wave.
The (M-sequence discrete) spread spectrum pseudo random signals are unique signals for respective ultrasonic measuring apparatuses. That is why even if an ultrasonic wave that has been sent out from another ultrasonic measuring apparatus reaches the receiver 9, its correlation with the pseudo random signal generated by the pulse generator 105 is very little. Consequently, no peak is detected by the correlator 103 and the ultrasonic measuring apparatus 101 can identify a pseudo random signal that has come from another ultrasonic measuring apparatus.
As described above, between M-sequences, signals other than the desired one look nothing but noise, and therefore, there is very little correlation with the received signal of an ultrasonic wave that was sent out from another ultrasonic measuring apparatus. That is why no peak of the correlation value is detected, and the pseudo random signal obtained from another ultrasonic measuring apparatus can be identified.
However, if such an ultrasonic measuring apparatus that uses the M-sequence coding, for example, is built in a moving body such as a self-moving robot or if a surrounding object is moving to the contrary, then there is a non-zero relative velocity between the surrounding object and the ultrasonic measuring apparatus. And if that relative velocity produces a Doppler shift, then a time lag will be caused between a correlation reference signal and the received signal during the correlation processing, thus decreasing significantly the M sequence's own correlation gain. FIG. 12 illustrates how that decrease in gain is caused. Specifically, FIG. 12(a) illustrates an M-sequence coded received signal 70. If there is no Doppler shift (i.e., if there is a zero relative velocity between the transmitter and the receiver), a despread waveform 71 such as the one shown in FIG. 12(b) is obtained as a result of the correlation processing. In the situation shown in FIG. 12(b), there is an outstanding peak of correlation at the time of reception 73 but the correlation noise in the ranges before and after the peak are so suppressed that a large SNR can be achieved. If a Doppler shift is produced, however, the peak of correlation will decrease significantly and the correlation noise in the ranges before and after the peak will also have raised levels. As a result, the SNR will drop steeply.
A method for compensating for such a Doppler shift is disclosed in Journal of Geography 110 (4), pp. 529-543 (2001). FIG. 13 is a block diagram showing the basics of the Doppler shift compensating method disclosed in Journal of Geography 110 (4), pp. 529-543 (2001). The basic system shown in FIG. 13 includes a number of despreading sections 81a through 81e with respective reference signals that can be spread or despread in a range in which a Doppler shift would be produced. In this system, the M-sequence coded received signal is processed by the respective despreading sections 81a through 81e, which output their despread waveforms 82a through 82e independently of each other. In the example illustrated in FIG. 13, the reference signal of the despreading section 81c agrees with the Doppler shift, and therefore, the despread signal 82c has an outstanding peak of correlation. Meanwhile, the despread signals 82b and 82d before and after the despread signal 82c have somewhat low peaks of correlation. But in the despread waveforms 82a and 82e in which the reference signal is spread or despread quite differently from the Doppler shift, only correlation noise is observed. According to the method of Journal of Geography 110 (4), pp. 529-543 (2001), the highest peak of correlation is selected from the multiple despread waveforms 82 and used to measure the propagation time, for example.
Another Doppler shift compensating method is disclosed in Japanese Patent Application Laid-Open Publication Nos. 2007-202088 and 2006-279173. FIG. 14 illustrates the fundamental principle of that Doppler shift compensating method disclosed in Japanese Patent Application Laid-Open Publication No. 2007-202088, which adopts an orthogonal frequency division multiplexing (which will be abbreviated herein as “OFDM”) method in which coding is carried out in a frequency range. Even such coding to be carried out in a frequency range is also significantly affected by a Doppler shift. When a Doppler shift is produced, the code that has been divided into the sub-carriers 91 shown in FIG. 14(a) will be spread or despread in the frequency range and will be received as the spread or despread sub-carriers 93 shown in FIG. 14(b). If such sub-carriers 93 are demodulated as they are, the original signal cannot be decoded. That is why according to the Doppler shift compensating method disclosed in Japanese Patent Application Laid-Open Publication No. 2007-202088, a Doppler shift compensating signal 92 is provided for a higher frequency range than the sub-carriers that should ordinarily be used for coding. As a result of the Doppler shift, the Doppler shift compensating signal 92 will turn into a signal 94 in FIG. 14(b) and a frequency shift 95 will be detected. According to Japanese Patent Application Laid-Open Publication No. 2007-202088, every sub-carrier 93 is re-sampled with the frequency shift 95 to restore the sub-carriers 96 of the original signal as shown in FIG. 14(c), and then the original signal is decoded by demodulating the sub-carriers 96. In the same way, according to the conventional technique disclosed in Japanese Patent Application Laid-Open Publication No. 2006-279173, the frequency shift of the sub-carriers 91 in the highest frequency range is also used as a Doppler shift compensating signal, and compensating for the Doppler shift of the sub-carriers as in Japanese Patent Application Laid-Open Publication No. 2007-202088.
The method disclosed in Non-Patent Document No. 1: Journal of Geography 110 (4), pp. 529-543 (2001) is a rather quick and secure Doppler shift compensating method. In addition, since no Doppler shift is supposed to be measured and then compensated for according to that method, it can also be used even when the received signal has produced multiple Doppler shifts. According to that method, however, it is still difficult to compensate for the Doppler shift perfectly and recover the correlation gain. That is why to ensure sufficiently high precision of measurement, the reference signal should be spread and despread at shorter intervals, which would require an increased number of despreading sections to provide. As a result, the size and cost of the hardware to use will rise too much to apply that technique to a consumer electronic device easily.
On the other hand, according to the method disclosed in Japanese Patent Application Laid-Open Publication Nos. 2007-202088 and 2006-279173, either a Doppler shift compensating signal is provided separately or a part of a signal is used as the Doppler shift compensating signal. In any case, the Doppler shift is estimated by using the Doppler shift compensating signal, and therefore, the compensation can get done rather accurately. According to such a method, however, the entire sub-carriers should be re-sampled and the signal should be recorded in its entirety. That is why to get the compensation done, a lot of memory space and processing time are needed. For that reason, if an acoustic wave is used as a carrier, the method disclosed in Japanese Patent Application Laid-Open Publication Nos. 2007-202088 and 2006-279173 is certainly applicable to low-speed data communications in which a relatively low processing rate is permitted. However, such a method should not be applied to processing that requires high speed response such as sensing an obstacle for a self-moving robot.
Furthermore, in sensing an obstacle for a self-moving robot, for example, generally there are an unspecified large number of objects to sense, which will include objects (such as persons) that move at various relative velocities. Consequently, as there will be a mixture of various relative velocities between the self-moving robot and the objects of sensing, it is difficult to apply the method disclosed in Japanese Patent Application Laid-Open Publication No. 2007-202088 that uses a compensating signal on an individual basis to such a situation.